1. Field of the Invention
The present invention relates to a communication device for a small vehicle. In particular, the present invention relates to a communication device such as an antenna for a two-wheeled vehicle or a three-wheeled vehicle.
2. Description of Background Art
In a motorcycle or other small vehicle, an antenna may intercept a field of view when it is located at the front of a vehicle body. Therefore, antennas in small vehicles are typically mounted to a rear portion of the vehicle body (for example, see JP-UM-A-63-91991).
The antenna disclosed in the JP-UM-A-63-91991 document is provided on a right side portion of a motorcycle behind the seat so as to project upward and rearward at a slightly oblique angle.
Although it is not entirely clear from the JP-UM-A-63-91991 document, the antenna is adapted to catch electric waves of a wireless communication device or electric waves from a radio station that have a relatively long wavelength and a low directivity. Therefore, a passenger such as a rider or nearby buildings does not interfere with these waves. In view of this, the antenna can be provided behind the passenger seated on the seat so as to project therefrom without any problem.
However, in the case of an antenna for transmitting signals between vehicles for detecting positions of the vehicles with respect to each other, carrier waves in an extremely high-frequency wave band that transmit a large amount of data are transmitted. The carrier waves are high in directivity and are therefore interfered with by a passenger or nearby buildings. In the case of electromagnetic waves such as light, the waves are further interfered with by a passenger or nearby buildings.
In particular, when a passenger interferes with electric wave reception, the range on the passenger side from the antenna becomes an incommunicatable range. Therefore, in order to enable communication in all directions, the antenna must be arranged at a higher position than the passenger. Alternatively, at least two antennas must be arranged at the front and rear, or the left and right positions of the vehicle body to avoid a lack of communication in some directions.
When an antenna is arranged at a higher position, an arrangement at the front, left, and right side of the vehicle body is a hindrance. Therefore, an antenna that is arranged at a higher position is typically arranged at the rear portion of the vehicle body.
Even when an antenna is arranged at a higher position at the rear portion of the vehicle body, the communication range is reduced due to the existence of nearby buildings on the left and right sides, for example, just before the vehicle enters into an intersecting road at a blind corner that is situated in an environment that has buildings on the left and right sides.